Mohamed E. Ali
Mechanical Engineering Department, King Saud University,
experimental investigations have been reported on steady state natural convection from the outer surface
of horizontal ducts in air. Five ducts have been used with aspect ratios (Γ= duct height/duct width) of 2, 1
and 0.5. The ducts are heated using internal constant heat flux heating elements. The temperatures along
the surface and peripheral directions of the duct wall are measured. Longitudinal (circumference
averaged) heat transfer coefficients along the side of each duct are obtained for laminar and transition
regimes of natural convection heat transfer. Total overall averaged heat transfer coefficients are also
obtained. Longitudinal (circumference averaged) Nusselt numbers are evaluated and correlated using the
modified Rayleigh numbers for transition regime using the axial distance as a characteristic length.
Furthermore, total overall averaged Nusselt numbers are correlated with the modified Rayleigh numbers,
the aspect ratio and area ratio for the laminar and transition regimes. The longitudinal or total averaged
heat transfer coefficients are observed to decrease in the laminar region and increase in the transition
region. Laminar regimes are obtained only at very small heat fluxes otherwise transitions are observed.